1. Field of the Invention
This invention relates to power amplifier systems and more particularly to power amplifier systems using feedback of distortion components in the system output signal to reduce distortion and improve frequency response over a wide band of frequencies.
2. Discussion of the Prior Art
Wideband amplifiers are useful in a broad variety of applications such as amplification of frequency division multiplexed communication signals. The wider the amplifier bandwidth, the fewer the number of amplifiers that may be required for a given application. Amplifiers which remain relatively linear over a broad range of frequencies are available for relatively non-demanding applications. However, when it is sought to use Class C operation for greater power output, higher gain, and greater power efficiency, it becomes much more difficult to provide linear and hence distortion free operation over a wide frequency range.
N. Sokal et al, U.S. Pat. No. 3,900,823 on "Amplifying And Processing Apparatus For Modulated Carrier Signals" issued Aug. 19, 1975 is exemplary of prior art devices. Devices such as those taught by Sokal et al attempt to linearly amplify amplitude modulated and single sideband and signals by detecting the modulating signal at the input and the output of an amplifier and developing a negative feedback signal from the difference to control the amplifier gain by controlling the input power supply voltage. Such an arrangement is not appropriate for wideband applications where several modulated carrier signals may be applied to the amplifier input simultaneously. Sokal et al also contemplate shifting the frequency or phase of the input to the amplifier system to compensate for non-linearities. They fail however to adequately provide for variable phase shifts resulting when frequencies are altered, as often occurs in wideband amplifier systems. Devices, such as the circuits presented in Sokal et al, provide adequate control of power amplifiers so long as the bandwidth of the signal being processed remains narrow so that the frequencies applied to the amplifier are relatively constant.
H. Stover, U.S. Pat. No. 3,810,018 on "Distortion Compensator For Phase Modulation Systems" issued May 7, 1974 is an example of an amplifier system with a distortion compensating circuit for phase modulation communication systems. Such a system incorporates fixed phase shifts and does not include a means for varying the phase shifts that are induced by frequency changes. Therefore the device will function satisfactorily only within a predetermined narrow frequency band.
In addition to the more common feedback linearization techniques, numerous attempts have been made to linearize amplifier systems through the use of feed-forward circuits. Many of the circuits incorporate phase and gain control means as well as delay networks but they also fail to compensate automatically for phase shifts arising from frequency variations. For instance, the below listed patents all incorporate fixed delay means in feedforward schemes which are calculated to compensate for phase shifts that occur between the feedforward signal and the primary signal as a function of the difference in transient time for the signal through the parallel circuits.
U.S. Pat. No. 3,471,798--Feedforward Amplifier, H. Seidel PA0 U.S. Pat. No. 3,541,467--Feed-Forward Amplifier With Frequency Shaping, H. Seidel PA0 U.S. Pat. No. 3,815,040--Feed-Forward, Error-Correcting Systems, H. Seidel PA0 U.S. Pat. No. 3,873,936--Apparatus For Reducing Distortion In A Repeated Transmission System, Yo-Sung Cho PA0 U.S. Pat. No. 3,886,470--Feed-Forward Amplifier System, W. O'Neill et al
W. Denniston et al, U.S. Pat. No. 3,922,617 on "Adaptive Feedforward System" issued Nov. 25, 1975 is a feed-forward system which includes feedback concepts calculated to enable the system to cope with varying circuit parameters. This feed-forward/feedback system compensates for varying parameters through the use of pilot signals which are of fixed frequency. Therefore the adaptive processes carried out by the circuitry provide adaptation for distortion created by uncontrolled variations in system components but fail to provide any adaption for wideband frequency variations in the signal being processed.